Re: [Boost-users] [mpl][variant] Visitor with typelists

Subject: Re: [Boost-users] [mpl][variant] Visitor with typelists
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-12-29 13:21:51

AMDG

Tilo Nitzsche wrote:
> What is the best (shortest/most elegant) way to create a visitor for a Variant, where there are subsets of the variant types, for which the operation is the same?
>
> typedef boost::mpl::vector<A, B, C> v1;
> typedef boost::mpl::vector<D, E> v2;
> typedef boost::mpl::joint_view<v1, v2> v;
> typedef boost::make_variant_over< v >::type v_union;
>
> struct visitor : public boost::static_visitor<int> {
>     int operator()([T in v1] arg) { // do something for A, B, C
>         return arg.a;
>     }
>     int operator()([T in v2] arg) { // do something for D, E
>         return arg.b;
>     }
> };
>   

Use enable_if

typedef boost::mpl::vector<A, B, C> v1;
typedef boost::mpl::vector<D, E> v2;
typedef boost::mpl::joint_view<v1, v2> v;
typedef boost::make_variant_over< v >::type v_union;

struct visitor : public boost::static_visitor<int> {
    template<class T>
    typename boost::enable_if<boost::mpl::contains<v1, T>, int>::type
    operator()(T arg) { // do something for A, B, C
        return arg.a;
    }
    template<class T>
    typename boost::enable_if<boost::mpl::contains<v2, T>, int>::type
    operator()(T arg) { // do something for D, E
        return arg.b;
    }
};

In Christ,
Steven Watanabe